There have been used various optical head devices for recording information in the information recording surface an optical recording medium such as an optical disk such as CD, DVD or the like, or a magneto-optical disk (hereinbelow, referred to as “the optical disk” as a whole), or reproducing the information in the information recording surface. A diffraction element is used for many purposes in such optical head device.
In the optical head device, the optical disk is rotated while a laser light is focused on a track formed in the information recording surface of the optical disk. Therefore, it is necessary not to deviate focused laser beams from the track. For this, various tracking methods have been developed. Among these tracking methods, a 3-beam method used for recording information is widely known. Further, a push-pull method used for recording information, i.e., a method using a light receiving element divided into two portions in parallel to a track to receive reflection light from an optical disk so that the difference between two divided portions of reflection light is taken, in particular, a differential push-pull method capable of canceling the offset of signals is widely known.
The three-beam method and the differential push-pull method are common in such points that a diffraction element is used, and the main beam as the 0th-order diffraction light and a sub-beam as ±1st-order diffraction lights are generated by the diffraction element.
In order to record or reproduce information in both optical disks of CD and DVD having different standards and structures, a CD-DVD compatible optical head device (hereinbelow, referred to as the compatible optical head device) has been noted in recent years. When a CD series optical disk such as CD-R is used in order to reproduce the information in the compatible optical head device, a semiconductor laser having a 790 nm wavelength band is used. Further, when a DVD series optical disk is used, a semiconductor laser having a 650 nm wavelength band is used for reproducing.
With reference to an example of the construction in FIG. 9, a first conventional optical head device having two semiconductor lasers arranged with a space will be described. Emission lights from semiconductor lasers 3A, 3B are synthesized on the same optical axis by means of a wavelength synthesizing prism 9, and the synthesized light transmits through a beam splitter 4. Then, the light is transformed into a parallel beam by a collimator lens 5, and it transmits through an objective lens 6 to be focused onto the information recording surface of an optical disk 7. The focused light is reflected at the information recording surface, and the reflected light traces reversely the same light path as the coming route.
Namely, the reflected light is transformed again into a parallel beam by the objective lens 6; is collected by the collimator lens 5 and enters into the beam splitter 4. Light reflected at the beam splitter 4 propagates along an optical axis extending with an angle of 90° with respect to the optical axis of the coming route, and is focused on a light receiving plane of a photodetector 8. The light as a signal is converted into an electrical signal by the photodetector 8. In FIG. 9, a 3-beam generating diffracting grating 10 is used for the light having a 790 nm (hereinbelow, it may be referred to as λ2) wavelength band.
As a semiconductor laser for emitting light having two wavelengths usable for such optical head device, a two-wavelength semiconductor laser of monolithic structure in which a semiconductor laser having a 790 nm wavelength band and a semiconductor laser having a 650 nm (hereinbelow, it may be referred to as λ1) wavelength band are formed in a single chip, for example, is proposed. Further, a two-wavelength semiconductor laser comprising a plurality of chips in which laser chips having different wavelength bands are disposed with an interval of light emitting point of about 100-300 μm, is proposed recently. By using such semiconductor lasers, the number of parts can be reduced, the size of the device can be reduced and cost for manufacturing can be reduced in comparison with conventional optical head devices having two semiconductor lasers as separate units as shown in FIG. 9. Accordingly, there is a strong demand for the 3-beam generating diffraction grating usable for the two-wavelength semiconductor laser.
A second conventional optical head device using a diffraction element is shown. In the optical head device for recording information in an optical disk, the returning light originated from emission light from a semiconductor laser, which is reflected at the optical disk, is introduced into a light receiving element as a photodetector through a beam splitter. A holographic diffraction element (a holographic beam splitter) is used as such beam splitter.
FIG. 11 is a diagram showing a conventional compatible optical head device using a holographic beam splitter (FIG. 11(a) shows a case of emitting light having a λ1 wavelength band and FIG. 11(b) shows a case of emitting light having a λ2 wavelength band). An emission light from a semiconductor laser 3 for emitting light having a 650 nm wavelength band and light having a 790 nm wavelength band is transformed into a parallel beam by a collimator lens 5, and the light is focused on a optical disk 7 by means of an objective lens 6. The reflection light from the optical disk 7 is passed again through the objective lens 6 and the collimator lens 5 to reach a light receiving element as a photodetector 8A (FIG. 11(b)) or 8B (FIG. 11(a)) through a holographic beam splitter. The light receiving element converts the received reflection light into an electrical signal. The electrical signal is amplified by an amplifier, and a gain is multiplied to the electrical signal in an automatic gain correction circuit whereby the electrical signal is adjusted to have a predetermined range of signal level. FIG. 11 omits the amplifier and the automatic gain correction circuit.
On the other hand, JP-A-4-129040 discloses a wavelength-selective diffraction element as a CD-DVD compatible diffraction element wherein the optical path difference is made to be integer times as much as the wavelength of either incoming light and the optical path difference is made to be non-integer times as much as the wavelength of the other incoming light. Explanation will be made as follows. The diffraction efficiency of a diffraction grating of two levels (in rectangular shape) in which a ridge and a bottom appear alternately can be expressed by the following approximation formulas where λ represents a wavelength, R represents an optical path difference, η0 represents the diffraction efficiency of the 0th-order light, ηm represents the diffraction efficiency of an mth-order light and m represents an integer other than 0.η0=[1+cos(2π×R/λ)]2/4ηm=[1−cos(2π×R/λ)]2×[1−(−1)m]2/(4πm2)
When the optical path difference is determined to be integer times as much as the wavelength of either incoming light, the above-mentioned formulas provide η0=1 and ηm=0. Further, when the optical path difference is non-integer times as much as the wavelength of the other incoming light, a wavelength-selective diffraction element providing 0<η0<1 and 0<ηm<1, can be obtained.
The explanation as to the case of the diffraction element having two levels has been made. However, a wavelength-selective diffraction element can be obtained even when it has the grating of multi-leveled shape, in particular, a pseudo-blazed shape as long as conditions that the optical path difference of one level (one step) is integer times as much as the wavelength of either incoming light and non-integer times as much as the wavelength of the other incoming light are satisfied.
An example of a conventional optical head device utilizing such wavelength-selective diffraction element is described.
FIG. 13 shows a third conventional optical head device in which the wavelength-selective diffraction element is used as an aperture limiting element. An aperture limiting element 18 is made of a glass substrate such as synthesized quartz glass. An example of the construction of a conventional wavelength-selective diffraction element used as the aperture limiting element is shown in FIG. 12. As shown in FIG. 12, a diffraction grating having an optical path difference which is two times as much as the wavelength λ1 of a DVD series optical disk, is formed only in a peripheral portion of the aperture limiting element 18. Then, the optical path difference is about 1.6 times as much as the wavelength λ2 of a CD series. Accordingly, it can transmit light having a wavelength λ1 and diffract at least 70% of the light having the wavelength λ2. In FIG. 12, the grating surface of the diffraction grating divided into two portions is for the reason that the light which is diffracted in both the coming and returning routes and propagates on the same light path as the transmitting light without being diffracted, should not be focused on the photodetector.
As shown in FIG. 13, emission lights from semiconductor lasers 3A, 3B are synthesized on the same optical axis by a wavelength synthesizing prism 9, and the synthesized light passes through a beam splitter 4. Then, the transmitting light is transformed into a parallel beam by a collimator lens 5 to be received by the aperture limiting element 18. Light of λ1 transmits without being diffracted through the peripheral portion (a shaded portion in FIG. 12(b)) and a central portion (an inner portion of a circle in FIG. 12(b)) of the aperture limiting element 18, and is focused on the information recording surface of a DVD series optical disk 7 by means of an objective lens 6 (FIG. 13(a)).
Further, light of λ2 is diffracted at the peripheral portion of the aperture limiting element 18 and only the light passing through the central portion is focused with a smaller aperture on the information recording surface of the optical disk 7 (FIG. 13(b)). The reflected light from the optical disk is again transmitted through the objective lens 6, the aperture limiting element 18 and the collimator lens 5 to be received by the beam splitter 4. The light reflected at the beam splitter 4 propagates along the optical axis extending with an angle of 90° with respect to the light axis of the coming route to be focused on the light receiving surface of a photodetector 8. Then, the light as a signal is converted into an electrical signal in the photodetector 8. Further, the light of λ2 diffracted by the aperture limiting element 18 and focused on the information recording surface of the optical disk 7 is reflected at the optical disk, and the reflected light propagates on the same light path as the signal light, and is focused on a portion other than the light receiving surface of the photodetector 8, although there is omission in FIG. 13(b).
FIG. 15 shows a fourth conventional optical head device in which the wavelength-selective diffraction element is used as a wavelength-selective deflection element. The wavelength-selective deflection element is made of a glass substrate such as synthesized quartz glass. FIG. 14 shows another example of the construction of the conventional wavelength-selective diffraction element. It shows a pseudo-blazed diffraction grating 19 of multi-levels such as 5-7 levels (4-6 steps) wherein the optical path difference R of one level (one step) is equal to the wavelength λ1 of the DVD series optical disk (although the grating of 6 levels (5 steps) is shown in FIG. 14, the number of levels should not be limited to such).
This diffraction grating can transmit light having the wavelength λ1 without being diffracted and diffract at least 60% of light having the wavelength λ2 in one of the diffraction orders so that the light is deflected.
As shown in FIG. 15, an emission light having the wavelength λ1 emitted from a luminescent point of a two-wavelength semiconductor laser 3 (FIG. 15(a)) and an emission light having the wavelength λ2 emitted from the other luminescent point are transformed into parallel beams by a collimator lens 5 after they have passed through a beam splitter 4, and the parallel beams are focused on the information recording surface of an optical disk 7 through an object lens 6.
Reflection lights from the optical disk 7 transmit again through the objective lens 6 and the collimator lens 5 and enter into the beam splitter 4 to be reflected. The reflected lights propagate along the optical axis extending with an angle of 90° with respect to the optical axis of the coming route and are incident into a wavelength-selective deflection element 19. The light having the wavelength λ1 entering into the wavelength-selective deflection element 19 transmits through the deflection element 19 without being diffracted, and is focused on the light receiving surface of the photodetector 8 (FIG. 15(a)). On the other hand, the light having the wavelength λ2 entering into the wavelength-selective deflection element 19 is deflected at the deflection element 19 and is focused on the same light receiving surface of the photodetector 8 as that for the light having the wavelength λ1 (FIG. 15(b)).
In the first conventional optical head device in which the 3-beam generating diffraction element used for the 3-beam method or the differential push-pull method is used in combination of the two-wavelength semiconductor laser, however, the following problem arises. Namely, the diffraction element has the diffraction effect to either an incoming light of a 790 nm wavelength band for a CD series optical disk or an incoming light of a 650 nm wavelength band for a DVD series optical disk whereby diffracted lights are produced. As a result, unwanted diffracted lights as stray lights enter into the photodetector, whereby it is impossible to record or reproduce the information. Further, there is also a problem that the diffraction grating provided to generate three-beams for an incoming light for either optical disk diffracts an incoming light for the other optical disk to produce unwanted diffraction light to thereby cause a loss of light quantity and reduce intensity of signals.
In order to solve such problems, the before-mentioned JP-A-4-129040 discloses the wavelength-selective diffraction element in which the optical path difference is made to be integer times as much as the wavelength of either incoming light and the optical path difference is made to be non-integer times as much as the wavelength of the other incoming light. However, the condition of making the optical path difference of an incoming light having a wavelength band to be integer times as much as the wavelength of the incoming light would restrict flexibility in designing the other wavelength, and flexibility in selecting the diffraction efficiency was also limited, and therefore, the disclosed diffraction element was unsatisfactory.
Further, in the case of the second conventional optical head device, namely, when the holographic beam splitter is used in combination of the two-wavelength semiconductor laser having a monolithic structure, the following problem results. Namely, when light having a wavelength λ is incident into a holographic beam splitter comprising a diffraction element having a pitch of grating P, sin θ is in proportion to λ/P where θ represents a diffraction angle of light. Accordingly, the diffraction angle of light having a 650 nm wavelength band is different from light having a 790 nm wavelength band because of their having different wavelengths. Therefore, it is necessary to increase the light receiving surface area when the diffraction lights are received by a single photodetector.
To increase the light receiving surface area causes deterioration of high frequency characteristics whereby a rapid reproduction of an optical disk is difficult. On the other hand, when light receiving surfaces are formed in the photodetector in order to receive the light having a 650 nm wavelength band and the light having a 790 nm wavelength band, the number of light receiving elements increases twice with the result of a problem that the signal processing circuit becomes complicated.
JP-A-2000-76689 discloses a method of solving this problem, wherein a diffraction grating whose optical path difference is equal to the wavelength λ1 of a DVD series is formed in a surface of a substrate, and a diffraction grating whose optical path difference is equal to the wavelength λ2 of a CD series is formed in another surface of the substrate so that signals can be detected by a single small-sized photodetector.
The before-mentioned formulas of diffraction efficiency are approximation formulas which satisfy only in a case that the pitch of a diffraction grating is considered to be very large in comparison with the optical path difference of the diffraction grating. Accordingly, when the optical path difference is larger or the grating pitch is smaller, these approximation formulas are not applicable. Even though the optical path difference is made to be n times (n: a natural number) longer than the wavelength, i.e., conditions of η0=1 and ηm=0 are given in these formulas, η0=1 is not actually established or ηm=0 may not be established. The incapability of establishing these approximation formulas is called a resonance of the diffraction grating, and when the grating pitch becomes smaller or the optical path difference becomes larger, the incapability of establishing these approximation formulas appears remarkably.
Usually, the grating pitch of the holographic beam splitter was small as 5 μm or less. Since the grating pitch was small as compared with the optical path difference, there was a problem of causing a reduction of the transmittance of a wavelength to be transmitted, due to the above-mentioned resonance.
In the third conventional optical head device, it was necessary to make the diffraction grating pitch smaller to increase the diffraction angle of the diffracted light so that an unwanted light of a wavelength λ2 diffracted by the aperture limiting element did not incident into the light receiving element for signal detection. However, when the diffraction grating pitch was made smaller as described above, the transmittance of the light of a wavelength λ1 expected to be transmitted was reduced to thereby reduce the characteristics of the aperture limiting element.
In the fourth conventional optical head device, it was necessary to increase the number of levels of the pseudo-blazed diffraction grating to 5 to 7 in order to allow transmitting the light of a wavelength λ1 and to diffract the light of a wavelength λ2. The increment of the number of levels resulted that the optical path difference in total was 4 to 6 times longer than the wavelength λ1, whereby the transmittance of the light of a wavelength λ1 expected to be transmitted was reduced, and the characteristics as the wavelength-selective diffraction element were reduced.
It is an object of the present invention to solve the above-mentioned problems and to provide a wavelength-selective diffraction element providing a large degree of freedom in designing, i.e., allowing optional determination of the diffraction efficiency; avoiding producing no optical path difference with respect to light of a wavelength expected to be transmitted, and causing no reduction of the transmittance of the light having a wavelength expected to be transmitted even though the plating pitch is small.
Further, the present invention is to provide an optical head device provided with such wavelength-selective diffraction element and a two-wavelength semiconductor laser to record and reproduce information stably.